Fragile X Syndrome (FXS) is caused by a triplet repeat expansion whose abnormal methylation silences expression of the Fragile X Mental Retardation protein, FMRP. Loss of function of this neuronal RNA- binding protein results in the intellectual disability, seizures and autistic features characteristic of FXS. FMRP is widely accepted to regulate translation of specific mRNAs and many forms of synaptic plasticity in neurons are dependent on its function. A missense mutation occurring in a Fragile X patient within an RNA binding domain of FMRP is sufficient to cause the disease and abolishes FMRP's polysome association, suggesting that identifying FMRP's mRNA targets and how FMRP regulates their translation is key to understanding the molecular basis for the cognitive and behavioral changes typical of the disease. A new technique designed to capture in vivo interactions of RNA binding proteins with their RNA targets, UV crosslinking- immunoprecipitation combined with high throughput sequencing (HITS-CLIP), was used to identify FMRP- interacting mRNAs in total brain polysomes including a mixture of many neuronal subtypes in different states of development or activity. FMRP was found to regulate mRNAs encoding both pre- and postsynaptic proteins important for synaptic function, suggesting that their mis-expression in the absence of FMRP might underlie defects in synaptic plasticity. This list of 842 FMRP targets is a resource for focusing research to ameliorate symptoms of FXS by inhibiting the activity of proteins that may be overexpressed in the absence of FMRP. To identify the subset of targets of FMRP that are most relevant for phenotypic defects this proposal aims to develop and apply an innovative approach to measuring cell-specific interactions of FMRP with RNA by engineering a new mouse (cTAG) which will conditionally tag FMRP from the endogenous locus in specific cells with temporal control by breeding with inducible Cre lines so that the tag can be used for HITS-CLIP. FXS arises due to loss of FMRP in the context of normal expression of its two family members, FXR1P and FXR2P, which share some functional redundancy with FMRP. This has led to the fundamental question of whether FXS arises due to loss of a function specific to FMRP and not shared by its paralogs, or whether FXS results from decreased dosage of a family of functionally redundant proteins. This proposal will test in vivo functional redundancy of the three paralogs with regard to RNA interactions, both globally and in specific neurons with FXR1/2P cTAG mice, and test whether increasing expression of FXR1/2P in the absence of FMRP can rescue phenotype. Successful accomplishment of the proposed Aims has significance for understanding the molecular basis of synaptic function as well as the human diseases that result from its dysfunction, including Fragile X Syndrome and autism. Identification of specific, phenotype-relevant mRNA targets of FMRP not shared by its paralogs will focus attention on inhibition of this subset while evidence for redundancy will spur attempts to therapeutically upregulate the levels of the FXR1/2P proteins in FXS.